1. Technical Field
This disclosure relates to a reference voltage generator employing a Digital to Analog (DA) converter and an output voltage variable DC-DC converter including the reference voltage generator, and more specifically, to a circuit that generates a reference voltage to be supplied to the DA converter.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of a conventional commonly-used reference voltage section employing a tracking DA converter (see, Japanese Patent Application Laid-Open Publication No. 2005-333523).
Referring to FIG. 1, a reference voltage generator 100 includes a reference voltage generating circuit 110 and a DA converter 120. The DA converter 120 includes a setting voltage register 121, a digital comparator 122, an up/down counter 123, a code-voltage conversion circuit 124, and an AND circuit 125. In addition, the reference voltage generator 100 receives a voltage setting signal Voset and a clock signal CLK, generates a reference voltage Vdao in accordance with the voltage setting signal Voset, and outputs the generated reference voltage Vdao.
The setting voltage register 121 stores the voltage setting signal Voset, which is a digital signal, output from a control circuit (not shown) and outputs as an output code signal a signal value of the voltage setting signal Voset to a digital comparator 122. The up/down counter 123 counts the number of the clock signals CLK input through the AND circuit 125, outputs as an output code signal the count result to the digital comparator 122 and the code-voltage conversion circuit 124. In addition, the up/down counter 123 carries out up-counting or down-counting in accordance with an up/down signal Su/Sd received from the digital comparator 122.
The digital comparator 122 compares the output code signal from the setting voltage register 121 and the output code signal from the up/down counter 123. When the output code signals are the same, the digital comparator 122 outputs a low level signal as an output signal DCout. When the output codes are different, the digital comparator 122 outputs a high level signal as the output signal DCout. In addition, the digital comparator 122 outputs a down signal Sd so that the count value of the up/down counter 123 is decreased, when the output code signal from the up/down counter 123 is greater than the output code signal from the setting voltage register 123. On the other hand, the digital comparator 122 outputs an up signal Su so that the count value of the up/down counter 123 is increased, when the output code signal from the up/down counter 123 is smaller than the output code signal from the setting voltage register 123.
The code-voltage conversion circuit 124 converts the output code signal output from the up/down counter 123 into a voltage and outputs the voltage. This output voltage is the reference voltage Vdao, which is an output voltage from the reference voltage generator 100.
The code-voltage conversion circuit 124 may be configured with N resisters (N: a positive integer) having a resistance value of R and N+1 resisters having a resistance value of 2R that are connected in a ladder configuration, as shown in FIG. 2. Alternatively, the code voltage conversion circuit 124 may be formed with N resisters having a resistance value R that are connected in series between an output terminal of the reference voltage generating circuit 110 (at the reference voltage Vrt) and the ground terminal GND, as shown in FIG. 3. The code voltage conversion circuits 124 shown in FIGS. 2 and 3 control switches S0 through SN in accordance with the output code signal from the up/down counter 123 in order to change a dividing ratio for dividing the reference voltage Vrt output from the reference voltage generating circuit 110, thereby generating the reference voltage Vado. Namely, the reference voltage generating circuit 110 generates and provides the code-voltage conversion circuit 124 with the reference voltage Vrt that is to be used by the code-conversion circuit 124 in order to generate the reference voltage Vado.
However, every time the output code signals of the up/down counter 123 are changed, noise is caused in the reference voltage Vrt output from the reference voltage generating circuit 110, and it takes a relatively long time until the reference voltage Vrt is stabilized, which results in a slow DA conversion. Namely, when the clock signal CLK is input to the up/down counter 123 and the output code signals of the up/down counter 123 are changed, several switches among the switches S0 through SN are operated. At this moment, a current iccd flowing from the reference voltage generating circuit 110 to the code-voltage conversion circuit 124 is relatively largely fluctuated. The magnitude of the fluctuation varies depending on the number of the switches that operate at a time and whether the switches are turned on or off.
The current iccd is also a load current for the reference voltage generating circuit 110. Because the reference voltage generating circuit 110 is generally provided in an integrated circuit (IC) and occupies a narrow area in the IC, a large current cannot flow through the reference voltage generating circuit 110. In addition, the reference voltage generating circuit 110 cannot respond to a quick change in the current iccd, because the reference voltage generating circuit 110 is designed in order to reduce consumption current as much as possible. Therefore, there is a problem in that quick and great changes in the current iccd leads to quick and great changes in the reference voltage Vrt.
Moreover, when the above reference generator is used in a DC-DC converter that employs an output voltage of the DA converter as the reference voltage and dynamically changes the output voltage by changing the reference voltage in accordance with the voltage setting signal from the control circuit, it takes a relatively long time until the reference voltage is stabilized at a target voltage after the reference voltage is changed in accordance with the voltage setting signal. Namely, there is a problem in that the output voltage of the DC-DC converter cannot be quickly changed.
Furthermore, an output current capacity in the reference voltage generating circuit 110 needs to be increased in order to eliminate the influence on the reference voltage Vrt from the changes in the current iccd. Additionally, a bias current to be used in an inner circuit needs to be increased in order to improve a response speed. However, such countermeasures cause problems in terms of a larger circuit area, which leads to an increased chip size, and a large consumption current.